The overarching aim of this proposal is to develop and disseminate magnetic resonance imaging (MRI) tools to identify and characterize neuronal microstructure in small animals. MRI is an ideal modality for such studies because it can be applied non-invasively in vivo or non-destructively ex vivo, and can provide 3D evaluation across the entire brain. We will develop and test standardized MRI pulse sequences across multiple vendor platforms and develop an easily produced and standardized quality-assurance phantom and associated imaging protocol. Automated and optimized image reconstruction and quantitative microstructural parameter mapping software will be developed and disseminated as open source. A rigorous algorithmic framework for planning statistically-optimized, multi-objective imaging protocols will be developed and implemented in open source multi-platform software. A series of experimental studies will establish probabilistic volumes of a variety of neuronal microstructural property maps in normal and abnormal rat and mouse brains, along with quantitatively evaluated histology of select cerebral white matter regions. Throughout the project, novel MRI methods will be developed and tested, a variety of state-of-the-art methods will be standardized and optimized, and comparisons of MRI data with histology will offer novel insights in the micro anatomical basis of MRI contrast in white matter.